Factors that influence the spectroscopic properties of 1,3-bis[p-substituted-(phenylamino)]squaraines

Article abstract of DOI:10.1016/j.dyepig.2016.03.012

The spectroscopic characteristics of a series of ten aminosquaraine dyes that differ in the type of chemical substituent at the p-position of the phenyl ring were investigated. The effect of solvent on the properties of the dyes in their ground and excite

Full text of DOI:10.1016/j.dyepig.2016.03.012

Dyes and Pigments 130 (2016) 226e232
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Factors that influence the spectroscopic properties of 1,3-bis[p-
substituted-(phenylamino)]squaraines
,
Janina Kabatc a *, Katarzyna Kostrzewska a, Łukasz Orzeł b
a UTP, University of Science and Technology, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
b Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Cracow, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The spectroscopic characteristics of a series of ten aminosquaraine dyes that differ in the type of chemical
substituent at the p-position of the phenyl ring were investigated. The effect of solvent on the properties
of the dyes in their ground and excited state is described. The absorption and emission properties,
fluorescence lifetime and rate constants for radiative and non-radiative deactivation processes are also
presented. The dyes displayed very short fluorescence lifetimes (about 3 ns) and dependence of the
radiative deactivation process on the solvent viscosity.
Received 8 September 2015
Received in revised form
18 January 2016
Accepted 10 March 2016
Available online 16 March 2016
© 2016 Elsevier Ltd. All rights reserved.
Keywords:
Aminosquaraine dye
Absorption and fluorescence spectra
Fluorescence lifetime
Radiative deactivation
Non-radiative deactivation
1. Introduction
based squaraines were described for the first time in 1966 by
Sprenger and Ziegenbein [12]. The treatment of 3,4-
Squaraine dyes, first synthesised in the mid-1960s, consist of an
oxocyclobutenolate core with aromatic or heterocyclic components
at both ends of the molecule [1,2]. Many squaraines, possess pol-
ymethine structures with amine and iminium terminals. For this
reason, these compounds are occasionally classified as cyanine
dyes. 3,4-Dihydroxycyclobut-3-ene-1,2-dione is a core reactant in
the synthesis of squaraine dyes [3,4]. Squaric acid is a diacid that
exhibits two acidic hydroxyl groups with pKa values of 0.54 and
3.48 as well as two highly polarised carbonyl groups [5]. This
unique structure provides not only versatile proton acceptor sites at
the carbonyl function, for hydrogen-bonding associations, but also
binding sites to metal ions and other compounds [6e8] through the
hydroxyl groups. The first synthesis of squaraine dyes was reported
by Treibs and Jacob in 1965 [9]. This reaction involved the
condensation of 3,4-dihydroxy-3-cyclobutene-1,2-dione with two
dihydroxycyclobut-3-ene-1,2-dione with two molar equivalents of
primary or secondary amines avoids the formation of symmetrical
squaraines, resulting in a double condensation reaction by the
amine [7,13], commonly referred to as aminosquaraines. Amino-
squaraines have also been synthesised by the isomerisation of
squaric acid 1,2-diamides [7].
Squaraine dyes normally exhibit intense absorption and often
fluorescence emission. They have attracted much attention from
the viewpoint of technological applications. Generally, the spec-
troscopic properties of squaraine dyes depend on the chemical
structure of the chromophore heterocyclic or aromatic component.
Thus, efforts have been made to develop new synthetic methods for
constructing squaraine-based chromophores. The search for such
chromophores is still relevant due to the important practical ap-
plications of squaraines, including xerographic devices utilising the
photoconductive properties of squaraine dyes, photovoltaic devices
employing the dyes as photosensitisers, dyeing photoinitiating
systems employing squaraines also as photosensitisers [14e16] and
biolabeling and chemosensory materials for analytical uses. Low
band-gap electroconductive materials are available by the poly-
merisation of squaraine skeletons, as well as supramolecular sys-
tems. Furthermore, it should be noted, that these dyes are attaining
an increased importance in the field of supramolecular chemistry.
equivalents of electron-rich aromatics such as
a-unsubstituted
pyrroles and 1,3,5-trihydroxybenzene under acidic conditions.
Since then there have been many squaraine dyes possessing
different heterocyclic or aromatic components [7,10,11]. Aniline-
* Corresponding author.
E-mail address: nina@utp.edu.pl (J. Kabatc).
http://dx.doi.org/10.1016/j.dyepig.2016.03.012
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

J. Kabatc et al. / Dyes and Pigments 130 (2016) 226e232
227
These dyes have been applied in squaraine-based molecular sen-
sors, self-assembly, in polymeric materials and biological applica-
tions [17,18]. The polymers containing squaraine moieties form a
new class of low bands gap materials. For example, poly-
aminosquaraines have been examined as conducting polymers [19].
Moreover, in 2011, the crystal structure and study, from a supra-
molecular perspective, of other squaraines, e.g. anilinosquaraines,
was described by Silva et al. [20].
The aminosquaraine dyes reported here were synthesised by the
condensation reaction of squaric acid and a two molar equivalent of
(p-substituted)anilines. A characteristic feature of these com-
pounds is the position of the nitrogen atoms in the system of
conjugated double bonds. The chemical structures of these ami-
nosquaraine dyes, in particular the type of substituent on the
phenyl rings, vary their excited state energetics and the electronic
properties [19e22]. For example, as it has been shown by Park et al.,
slight modification of the squaraine structure by incorporating the
aryl amine functional group directly through the nitrogen atom of
the amino group to the squarate ring system results in drastically
different optical responses [23]. Thus, the objective of these studies
was the synthesis of a range of 1,3-aminosquaraine dyes with
differing structures, and investigation of their light absorption and
fluorescence properties. An understanding of the influence of
substituents in the p-position of the phenyl rings on the excited
state processes is very important in determining the photosensi-
tivity of these squaraine dyes, which is presented also in this article.
and Aref are the absorbances of dye and reference at the excitation
wavelength, Idye and Iref are the integrated emission intensity for
dye and reference, ndye and nref are the refractive indexes of the
solvents used to dissolve dye and reference, respectively.
Fluorescence lifetimes were measured using a single-photon
counting UVeVISeNIR Fluorolog 3 Spectrofluorimeter (Horiba
Jobin Yvon). The apparatus utilises, for excitation, a picosecond
diode laser generating pulses of about 55 ps at 370 nm. Short laser
pulses in combination with a fast microchannel plate photodetector
and ultrafast electronics make a successful analysis of fluorescence
decay signals with a resolution of few picoseconds possible. Dyes
were studied at a concentration able to provide the equivalent
absorbance at 370 nm (0.2 in the 10 mm cell) to be obtained.
Fluorescence decay was fitted to two exponentials.
3. Results and discussion
Ten aminosquaraines with structures presented in Fig. 1 were
studied and have been described here.
The general procedure of synthesis of aminosquaraines was
based on the condensation reaction of squaric acid with p-
substituted aniline derivatives in a mixture of 1-butanol and
toluene described in details by Part et al. [23].
The aminosquaraine dyes studied belong to a class of symmetric
DeAeD (donoreacceptoredonor) compounds, and differ in the
type of substituent in the p-position of the phenyl ring. The DeAeD
arrangement of each dye molecule has an interesting effect on the
formation of intramolecular charge-transfer states. Studies by
Kamat et al. [21] on the photochemistry of 1,3-bis[4-(dimethyla-
mino)phenyl]squaraine, and associated quantum chemical calcu-
lations, may suggest that 1,3-bis[p-substituted(phenylamino)]
squaraines are highly polarised, with the p-substituted-phenyl-
imino moiety being an electron donor (D) and the central C4O2 unit
being an electron acceptor.
It is well known that the properties of ground and excited state
compounds depend on the properties of the surrounding solvent.
The molecules of a solvent may interact (electrostatic interactions,
hydrogen bonding associations, Van der Waal interactions, etc.)
with molecules of dissolved compounds. It is also well known that
squaraines form a soluteesolvent complex in organic solvents; the
equilibrium constant of which is dependent on the DeAeD charge-
transfer character of the particular squaraine [21]. Therefore, the
effects of physicochemical properties of solvent on the properties of
each aminosquaraine compound shown in Fig. 1, in its ground and
excited state, were studied. The characteristic spectroscopic prop-
erties of each aminosquaraine dyes in a few solvents were also
studied. The solvents used differed in both polarity and viscosity.
The absorption and emission spectra of the dyes under inves-
tigation in acetonitrile are shown in Figs. 2 and 3.
2. Experimental
2.1. Materials and general methods
Squaraine dyes were synthesised by reaction of squaric acid
with aniline and p-substituted anilines, by the general method
described in the literature [23]. All reagents and solvents (spec-
troscopic grade) were purchased from Aldrich (Poland) and used
without further purification.
Absorption and emission spectra were recorded at room tem-
perature using an Agilent Technology UVeVis Cary 60 Spectro-
photometer, a Hitachi F-7000 spectrofluorimeter and UVeVISeNIR
Fluorolog 3 Spectrofluorimeter (Horiba Jobin Yvon), respectively.
Spectra were recorded in the following solvents: water (H2O),
dimethylsulfoxide (DMSO), acetonitrile (CH3CN), N,N-dime-
thylformamide (DMF), 1-methyl-2-pyrrolidinone (MP), methanol
(MeOH), ethanol (EtOH), acetone, tetrahydrofuran (THF) and
diethyl ether. The final concentration of dye in solution was
1.0 ꢀ 10ꢁ5 M. Spectroscopic measurements were performed in the
above mentioned solvents containing 20% of 1-methyl-2-
pyrrolidinone. For this purpose a suitable amount of dye was dis-
solved in 1-methyl-2-pyrrolidinone, then 2 mL of the concentrated
(ca. 1 mM) stock solution was added to a 10 mL volumetric flask
containing spectroscopic grade solvents.
The spectral characteristics of the aminosquaraine dyes are
summarised in Table 1.
The aminosquaraines under investigation have intense, sharp
absorption bands in the visible region with absorption maxima
ranging from 340 nm to 530 nm, and molar extinction coefficients
The fluorescence quantum yield for each dye in the solvents was
determined as follows. The fluorescence spectrum of a diluted dye
solution (A z 0.1 at 366 nm) was recorded by excitation at the
maximum of the absorption band of the standard. Dilute Coumarin
varying from 0.16 ꢀ 104
M
ꢁ1 cmꢁ1 to 11.3 ꢀ 104
M
ꢁ1 cmꢁ1. These
molar extinction coefficients are low in comparison with most
other squaraine dyes, but are still moderate to high in comparison
with simple azo dyes. The position of the absorption band
maximum and value of molar extinction coefficient for each dye
depends on the dye structure and properties of solvent used. The
majority of all aminosquaraine dyes tested, show an intensive
yellow colour in all solvents used, with the exception of dye SQ5,
which possesses a strong electron-withdrawing nitrogen group in
the p-position of the phenyl rings. The absorption band of SQ5 is
red-shift in comparison to the other dyes studied (see data in
I (
Ф
¼ 0.64 [25]) was used as a reference. The fluorescence spectra
of Coumarin I was obtained by excitation at 366 nm. The fluores-
cence quantum yield of each dye (fdye) was calculated using
equation (1):
n2dye
IdyeAref
fdye ¼ fref
,
,
(1)
n2ref
Iref Adye
where: fref is the fluorescence quantum yield of the reference, Adye

228
J. Kabatc et al. / Dyes and Pigments 130 (2016) 226e232
O
O
O
O
O
NH
NH
NH
NH
NH
Br
NH
H5C2
NH
NH
NH
NH
Br
C2H5
O
SQ1
SQ2
SQ3
O
O
O
NH
NH
C2H5O
NH
NO2
NH
OC2H5
NO2
Cl
Cl
O
O
O
SQ4
SQ5
SQ6
O
O
O
NH
HO3S
NH
SO3H
H3C
CH3
NH
I
NH
I
O
O
O
SQ7
SQ8
SQ9
O
NH
HO
NH
OH
O
SQ10
Fig. 1. The chemical structures of 1,3-bis-[p-substituted(phenylamino)]squaraines.
SQ1
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
SQ8
SQ9
SQ10
SQ1
SQ2
SQ3
SQ5
SQ6
SQ7
SQ9
SQ9
SQ10
1,00
0,75
0,50
0,25
0,00
1,00
0,75
0,50
0,25
0,00
360
420
480
540
600
660
420
480
540
600
660
Wavelength [nm]
Wavelength [nm]
Fig. 2. Absorption spectra of aminosquaraine dyes in acetonitrile.
Fig. 3. Fluorescence spectra of squaraine dyes in acetonitrile, lEX 380 nm and 490 nm
for SQ5.
Table 1).
It should be noted that in solution aminosquaraine dyes exist in
spectroscopic characteristics of dye SQ5, which possesses the
strong electron-withdrawing substituent (NO2) in the phenyl rings,
are somewhat different. Deprotonation to the monoanion is much
easier, and in some solvents (DMF for example) may occur to some
extent without the addition of any base [23], because the solvent
has sufficient Lewis basicity. Fig. 4 shows the effect that the Lewis
acidity/basicity parameters of the solvents on the electronic ab-
sorption spectra of SQ5.
two zwitterionic resonance forms. These compounds may also
undergo deprotonation, giving monoanion and/or dianions. The
deprotonation of the dye strongly depends on the type of substit-
uent in the p-positions of the phenyl rings. Introduction of electron-
withdrawing groups resulted in mono- or dianion formation. The
aminosquaraine dyes without electron-withdrawing substituents
exist as zwitterionic forms. It should also be noted that the mon-
oanion of aminosquaraines possess a broader absorption band, in
comparison with the zwitterionic form. Therefore, the
From the data presented in Fig. 4, and in Table 2, it can be seen
that a solution of SQ5 in a Lewis base solvent leads to the

J. Kabatc et al. / Dyes and Pigments 130 (2016) 226e232
229
Table 1
Spectral characteristics of 1,3-bis-[p-substituted(phenylamino)]squaraines.
Solvent
H2O
DMSO
MeCN
DMF
MP
MeOH
EtOH
Acetone
THF
Ether
SQ1
lab [nm]
ε [104 Mꢁ1cmꢁ1
lfl [nm]
374
0.304
504
13729
6900
4.3
0.167
3.859
378
0.555
504
5857
6614
11.0
0.448
3.856
390
0.36
496
7082
5480
5.4
0.208
3.858
393
0.76
502
4179
5525
28.0
1.088
3.805
393
0.19
495
4831
5243
46.0
1.798
3.843
390
11.0
498
7297
5561
51.0
1.978
3.841
393
11.2
492
7684
5120
6.7
0.26
3.858
398
11.3
490
7554
4717
8.0
0.31
3.858
401
0.69
477
14353
3973
10.0
0.402
3.857
400
0.53
484
8853
4339
7.4
0.287
3.858
]
FWHMfl [cmꢁ1
]
Stokes shift [cmꢁ1
]
]
]
]
]
]
]
]
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ2
]
lab [nm]
398
0.558
506
3547
5363
7.62
0.283
3.714
405
3.615
508
7112
5006
7.69
0.286
3.714
402
3.954
508
8527
5190
9.34
0.347
3.713
401
3.374
477
7455
3973
6.77
0.252
3.715
405
3.03
490
7661
4283
34.46
0.128
3.704
400
4.727
505
8673
5198
27.09
0.101
3.707
402
4.477
511
8159
5306
30.32
0.113
3.706
407
3.954
477
9186
3605
20.61
0.766
3.710
410
4.674
522
7600
5233
11.39
0.424
3.713
415
4.948
520
7411
4865
5.57
0.207
3.715
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
Stokes shift [cmꢁ1
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ3
]
lab [nm]
341
2.968
488
4821
8834
15.41
0.863
5.594
404
4.771
508
5928
5067
7.015
0.393
5.598
396
5.068
493
8408
4969
26.21
1.468
5.588
401
5.358
499
7107
4898
27.77
1.556
5.587
405
4.421
499
6548
4651
34.08
1.909
5.583
394
5.93
492
8568
5056
31.69
1.775
5.584
397
6.165
495
7894
4987
7.28
0.408
5.598
404
5.293
487
405
5.969
499
7475
4651
8.41
0.471
5.597
405
6.083
497
7531
4570
6.63
0.371
5.598
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
e
Stokes shift [cmꢁ1
4219
24.42
1.368
5.588
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ4
]
lab [nm]
340
1.973
499
4787
9372
11.43
1.358
11.863
408
3.429
499
6123
4470
21.84
2.593
11.850
407
3.408
495
4495
4368
38.69
4.595
11.831
397
0.622
498
6345
5109
7.84
0.932
11.867
410
2.772
495
6076
4188
35.43
4.208
11.834
400
3.541
497
5991
4879
33.39
3.966
11.837
403
3.443
497
5955
4639
48.11
5.714
11.819
407
1.093
482
4219
3823
65.82
7.818
11.798
409
4.061
497
6481
4329
10.47
1.244
11.864
409
1.659
490
6729
4042
9.21
1.094
1.866
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
Stokes shift [cmꢁ1
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ5
]
lab [nm]
399
1.339
452
4253
2939
0.971
17.784
1831.5
523
2.414
e
487
2.524
571
1669
3021
1.65
30.286
1831.4
522
2.421
562
530
2.269
621
2755
2765
1.97
36.127
1831.3
454
3.595
516
3513
2647
3.36
61.464
1831.1
470
2.869
545
465
4.886
536
474
2.207
532
1291
2300
1020.2
18686
1644.8
470
2.686
524
1226
2193
465.6
8528.4
1746.4
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
e
e
e
e
Stokes shift [cmꢁ1
e
1363
2.31
42.279
1831.2
2928
2.48
45.449
1831.2
2849
1.64
29.996
1831.4
ffl [ ꢀ 104]
3.09
56.672
1831.1
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ6
]
lab [nm]
342
1.308
507
4505
9516
13.12
0.668
5.085
404
3.311
515
7223
5335
8.24
0.419
5.087
399
3.367
516
8680
5683
4.91
0.250
5.089
399
2.833
515
10709
5645
15.98
0.814
5.083
403
2.395
510
8783
5206
32.16
1.637
5.075
398
4.076
509
9065
5479
15.00
0.764
5.084
400
3.887
510
8397
5392
17.96
0.914
5.083
409
4.619
476
9650
3441
12.99
0.661
5.085
410
4.141
519
7812
5122
14.95
0.761
5.084
412
4.380
531
7594
5439
9.04
0.460
5.087
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
Stokes shift [cmꢁ1
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ7
]
lab [nm]
349
2.353
496
3811
8492
14.24
0.488
3.421
407
4.097
500
6369
4570
8.88
0.304
3.423
399
2.585
478
7498
4142
12.89
0.442
3.421
403
4.097
477
10484
3849
8.79
0.301
3.423
409
1.673
477
7590
3486
34.13
1.169
3.414
402
2.439
506
6930
5113
10.27
0.352
3.422
404
3.835
507
7365
5029
25.45
0.872
3.417
407
2.318
499
6068
4530
13.05
0.447
3.421
413
4.102
514
6955
4758
11.40
0.391
3.422
418
4.132
511
6845
4354
11.20
0.384
3.422
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
Stokes shift [cmꢁ1
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ8
]
lab [nm]
393
0.737
478
14229
4525
10.45
0.371
3.549
408
0.373
478
5903
3589
20.42
0.371
3.546
406
0.318
475
6020
3578
18.74
0.725
3.546
408
0.804
478
5297
3589
19.14
0.666
3.546
410
0.490
472
6890
3204
34.46
0.680
3.541
402
0.482
475
6307
3823
12.44
1.224
3.548
402
0.461
477
6276
3911
16.19
0.442
3.547
374
0.331
476
8562
5730
22.21
0.575
3.545
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
Stokes shift [cmꢁ1
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
]
(continued on next page)

230
J. Kabatc et al. / Dyes and Pigments 130 (2016) 226e232
Table 1 (continued )
Solvent
H2O
DMSO
MeCN
DMF
MP
MeOH
EtOH
Acetone
THF
Ether
SQ9
lab [nm]
352
1.560
504
4780
8568
26.66
1.098
4.108
401
3.994
497
7095
4817
7.81
0.322
4.115
395
3.476
510
7984
5709
162.9
6.712
4.052
402
3.959
519
7228
5608
7.99
0.329
4.115
404
3.441
517
6818
5410
32.72
1.348
4.106
394
4.459
513
9224
5888
57.26
2.359
4.096
397
4.551
514
8719
5734
42.81
1.763
4.101
402
2.219
520
404
4.612
523
8079
5632
27.32
0.348
4.116
406
4.642
529
7951
5727
5.90
0.243
4.117
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
e
Stokes shift [cmꢁ1
]
5645
8.45
1.125
4.108
ffl [ ꢀ 104]
kr [106 sꢁ1
]
knr [108 sꢁ1
SQ10
]
lab [nm]
374
0.743
e
410
3.662
513
6215
4897
9.42
0.383
4.066
408
3.805
513
7572
5017
3.22
0.131
4.068
402
3.866
507
6820
5152
6.17
0.251
4.067
412
3.379
497
6385
4151
38.44
1.564
4.054
398
4.012
520
6921
5895
3.65
0.148
4.068
403
4.076
518
6595
5509
5.04
0.205
4.067
407
3.907
477
7554
3606
13.84
0.563
4.064
410
4.160
513
6514
4897
7.45
0.303
4.066
410
4.184
535
7560
5699
9.04
0.368
4.066
ε [104 Mꢁ1cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
e
Stokes shift [cmꢁ1
]
e
ffl [ ꢀ 104]
e
kr [106 sꢁ1
]
0.327
4.066
knr [108 sꢁ1
]
Table 2
DMSO
DMF
MP
1,00
0,75
0,50
0,25
0,00
Fluorescence lifetimes and fluorescence quantum yields of squaraines studied in 1-
methyl-2-pyrrolidinone.
Acetone
Dye
Fluorescence lifetime [ns]
t1
t2
ffl [ ꢀ 104]
SQ1
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
SQ8
SQ9
SQ10
2.60
2.69
1.79
0.84
0.59
1.96
2.92
2.81
2.42
2.46
10.39
10.76
7.14
3.37
e
46
34.46
34.08
35.43
1.97
32.16
34.13
34.46
32.72
38.44
7.86
11.68
11.26
9.71
9.83
400
500
600
700
directly populated due to the intersystem crossing quantum yields
being poor (FISC < 0.001) [25e28]. The excited triplet state of a
squaraine dye can only by generated by tripletetriplet sensitisation
[21].
Wavelength [nm]
Fig. 4. Effect of acidity/basicity parameters of solvents on the electronic absorption
spectra of 1,3-bis-(4-nitrophenylamino)squaraine.
The position of the fluorescence emission maxima for the ami-
nosquaraines under investigation depends both on the structure of
the dye and the type of solvent used with maxima ranging from
472 nm to 621 nm, respectively for SQ8 and SQ5 in MP. It is known
from Kamat et al. [21,22] that the emission spectrum consists of
three emission bands that arise as a result of free dye, dyeesolvent
complex, and a twisted excited state resulting from CeN bond
rotation. In the present study the spectral resolution is limited, to
resolve the emission bands at room temperature. The first emission
band corresponds to the sum of emissions from both free dye and
dyeesolvent complex (Fig. 6). The second emission band in the red
region (~470 nm for dye SQ8 and ~515 nm for dye SQ6), is seen as a
result of emission of a twisted excited state, resulting from CeN
bond rotation (see Fig. 6) [21].
broadening of the absorption band, in comparison to the absorption
band observed in acetone. Solvent basicity of the Lewis bases pre-
sented in Fig. 4, determined by Donor Number (DN), equals 29.8,
27.3 and 26.6 for DMSO, MP and DMF respectively. The application
of Lewis basic solvents to SQ5 results in the deprotonation of the
aminosquaraine, leading to monoanion formation (Fig. 5) [23,24].
The dyes under investigation show a blue shift in their respec-
tive absorption bands with an increase of solvent polarity, and the
lowest fluorescence emission in protic solvents (water for example)
with each excited state decaying non-radiatively. Basing on the
results described by Park et al. it is known, that the strong hydrogen
bonding solvents associate with the oxygen lone pair and/or ni-
trogen lone pair of each dye molecule reduces the magnitude of the
intramolecular charge-transfer associations within the dye mole-
cules [23].
The
fluorescence
quantum
yield
of
1,3-bis[p-sub-
stituted(phenylamino)squaraines is very low and also depends on
the dye structure and polarity of solvents. The highest fluorescence
quantum yield (given in Table 1) equals 0.102 for SQ5 in tetrahy-
drofuran, and the lowest being 0.000097 for SQ5 in water. These
fluorescence quantum yields observed experimentally are slightly
less than the reported value for 1,3-bis[4-(dimethylamino)phenyl]
squaraine [21,29].
Irradiation of a dye with visible light leads to an excited singlet
state formation. The dye in its excited state undergoes different
deactivation processes. Generally, they are radiative or non-
radiative processes. Fluorescence and phosphorescence are radia-
tive processes but internal conversion and intersystem crossing are
non-radiative processes. The triplet state of squaraines cannot be
To understand the excited state dynamics, the fluorescence

J. Kabatc et al. / Dyes and Pigments 130 (2016) 226e232
231
O
O
O
O
O2N
NH
NH
NO2
O2N
NH
NH
NO2
Resonanse structures
O
O2N
NH
N
NO2
O
Monoanion structure
Fig. 5. The resonance structures and monoanion of 1,3-bis-(4-nitrophenylamino)squaraine, respectively.
decay lifetimes of the dyes under investigation were measured.
Fluorescence lifetimes ( ) have been determined by single photon
t
counting techniques. The fluorescence decay function is a double
exponential when monitoring the fluorescence decay at 550 nm, as
it is shown in Fig. 7.
The fluorescence lifetimes measured for all dyes in MP are
summarised in Table 2.
As observed from the data presented in Fig. 7, and Table 2, the
fluorescence lifetime for 1,3-bis[p-substituted-(phenylamino)]
squaraines in MP solution is very short and equals about 3.0 ns. The
fluorescence quantum yield correlates very well with fluorescence
lifetime for almost all of the dyes studied. In MP the fluorescence
quantum yields and fluorescence lifetimes achieved similar values
for all dyes, with the exception of SQ5. The introduction of a strong
electron-withdrawing group (NO2) to the aminosquaraine mole-
cule resulted in about 5 times lower fluorescence lifetime and
about 15 times decrease in fluorescence quantum yield, in com-
parison to the other dyes studied. This suggests that the chemical
substituent changes bring instability for the excited state of SQ5.
With reference to studies by Law et al. [22], it can be concluded
that the longer fluorescence lifetimes may be considered as the
lifetime of the first singlet excited state of the most stable
conformer of the aminosquaraine dye molecule.
Fig. 7. Fluorescence decay curves of aminosquaraine dyes in 1-methyl-2-pyrrolidinone
as a solvent, lEX ¼ 370 nm, lEM ¼ 550 nm. Inset: The raport from measurements.
increase in fluorescence quantum yield could be related to dye
structure. The first group of dyes possess eH, -alkyl and ealkoxy
groups in the p-position of the phenyl ring. The second group
included chromophores with electron-withdrawing substituents
(eCl, eI, eBr, eSO3H) or an hydroxyl group. This behaviour was the
result of interaction between the polarised dye molecule in excited
state and the solvent [30]. In most of the dyes (except SQ5), the
fluorescence quantum yield achieved the lowest values in very low
polarity solvents (e.g. THF and diethyl ether).
The next investigation covered whether or not the fluorescence
quantum yield was sensitive to the solvent.
Inspection of data presented in Table 1 showed that the fluo-
rescence quantum yield increases with decrease of solvent polarity
(from water to MeOH and from water to MP for SQ1, SQ3, SQ4, SQ9
and for SQ2, SQ6, SQ7, SQ8 and SQ10, respectively). The observed
Taking into account both the fluorescence lifetime (t) and
Twisted excited state
fluorescence quantum yield (ffl), the rate constants of radiative (kr)
and non-radiative deactivation (knr) processes have been calculated
using eq. (2) and eq. (3).
1,00
0,75
ffl
kr ¼
(2)
(3)
t
Free-dye and
dye-solvent complex
0,50
ꢀ
ꢁ
1 ꢁ ffl
knr
¼
t
SQ8 DMSO
0,25
SQ6 DMF
SQ8 MP
SQ8 Ethanol
The values of radiative and non-radiative rate constants are
summarised in Table 1.
Apart from SQ5 (possessing a nitro group), for which the rate
constants of radiative and non-radiative deactivation are of the
0,00
420
450
480
510
540
570
600
order of 108 sꢁ1 and 1011 ꢁ1 respectively, the highest rate constants
s
Wavelength [nm]
for radiative and non-radiative processes were found for SQ4 (i.e.
ethoxy group), above 106 sꢁ1 and 1.18 ꢀ 109 sꢁ1 respectively. The
rate constants for radiative and non-radiative deactivation for the
Fig. 6. Sum of emission of both free dye and dyeesolvent complex and emission of a
twisted excited state resulting from CeN bond rotation, respectively.

232
J. Kabatc et al. / Dyes and Pigments 130 (2016) 226e232
References
2,0E+06
1,5E+06
1,0E+06
5,0E+05
0,0E+00
5,0E+08
4,5E+08
4,0E+08
3,5E+08
[1] Bigelow RW, Freund H. An MNDO AND CNDO/S(S þ DES CI) study on the
structural and electronic properties of a model squaraine dye and related
cyanine. J Chem Phys 1986;107:159.
SQ6
SQ7
[2] Beverina L, Sassi M. Twists and turns around a square: the many faces of
squaraine chemistry. Synlett 2014;25:477e90.
[3] Yagi S, Nakazumi H. Squarylium dyes and related compounds. Top Heterocycl
Chem, vol. 14. Berlin Heidelberg: Springer-Verlag; 2008. p. 133e81.
[4] Seitz G, Imming P. Oxocarbons and pseudooxocarbons. Chem Rev 1992;92:
1227e60.
0,0
0,5
1,0
1,5
2,0
Viscosity [mPa*s]
[5] Swartz LM, Howard LO. Conductance study of squaric acid aqueous dissocia-
tion. J Phys Chem 1971;75:1798e803.
[6] Terao H, Sugawara T, Kita Y, Kaho E, Takeda S. Proton relay in a one-
dimensional hydrogen-bonded chain composed of water molecules and a
squaric acid derivative. J Am Chem Soc 2001;123:10468e74.
[7] Schmidt AH. Reaktionen von quadratsaure und quadratsaure-derivaten.
Synthesis 1980;12:961e94.
SQ6
SQ7
[8] West R, Niu HY. New aromatic anions. VII. Complexes of squarate1 ion with
some divalent and trivalent metals. J Am Chem Soc 1963;85:2589e90.
[9] Treibs A, Jacob K. Cyclotrimethine dyes derived from squaric acid. Angew
Chem Int Ed 1965;4:694.
0,0
0,5
1,0
1,5
2,0
Viscosity [mPa*s]
[10] Ziegenbein W, Sprenger HE. Condensation products of squaric acid and azu-
lenic hydrocarbons. Angew Chem Int Ed 1966;5:893e4.
[11] Sreejith S, Carol P, Chithra P, Ajayaghosh A. Squaraine dyes: a mine of mo-
lecular materials. J Mater Chem 2008;18:264e74.
[12] Sprenger HE, Ziegenbein W. Condensation products of squaric acid and ter-
tiary aromatic amines. Angew Chem Int Ed 1966;5:894e5.
Fig. 8. The radiative rate constants as a function of solvent viscosity calculated for dyes
SQ6 and SQ7, respectively. Inset: The non-radiative rate constants as a function of
solvent viscosity.
[13] Pirrung MC, Han H, Chen J. O-Alkyl hydroxamates as metaphors of enzyme-
bound enolate intermediates in hydroxy acid dehydrogenases. Inhibitors of
isopropylmalate dehydrogenase, isocitrate dehydrogenase, and tartrate de-
hydrogenase(1). J Org Chem 1996;61:4527e31.
remaining aminosquaraines were about two orders of magnitude
lower than that observed for SQ5.
[14] He Y, Zhou W, Wu F, Miaozhen Li, Wang E. Photoreaction and photo-
polymerization studies on squaraine dyes/iodonium salts combination.
J Photochem Photobiol A Chem 2004;162:463e71.
[15] Lynch DE, Geissler U, A B. An investigation into the electrical conduction
properties of poly(oligo(1-methylpyrrol-2-yl)squaraine)s. Synth Met
2001;124:385e91.
The rate constant for the radiative process depended on the
viscosity of the solvent, as shown in Fig. 8. Alternatively, the rate
constant of non-radiative deactivation does not depend on the
viscosity of the solvent.
It can therefore be concluded that the electronic structure of the
fluorescence state is sensitive to the environmental perturbation
induced by the solvent. Inspection of Fig. 8 suggests that the value
of radiative rate constant increases with an increase in the viscosity
of the solvent. In the deactivation process of the excited singlet
state, the radiative relaxation process dominates, although in
addition to this process, the intramolecular rearrangements, for
example solvation phenomena as well as other specific kinds of
soluteesolvents interactions, might also play a role [30,31].
Furthermore, an excited singlet state may be self-quenched or
quenched by other molecule in a bimolecular reaction. A diffusion-
controlled self-quenching process has been observed for several
[16] Law KY, Facci JS, Ballry FC, Yanus JF. The conformational analysis of an
indolenium squaraine dye by DFT-GIAO calculation. J Imag Sci 1990;34:31.
[17] Mc Ewen JJ, Wallace KJ. Squaraine dyes in molecular recognition and self-
assembly. Chem Commun 2009:6339e51.
[18] Lynch DE. Pyrrolyl squaraines e fifty golden years. Metals 2015;5:1349e70.
[19] Tol AJW. Using symmetry forbidden interactions to create small band gap
polymers: polyaminosquaraine and related compounds.
J Chem Phys
1994;100:8463e70.
[20] Silva CE, Dos Santos HF, Speziali NL, Diniz RC, de Oliveira LF. Role of substit-
uent effect over the squarate oxocarbonic ring: spectroscopy, crystal struc-
ture, and density functional theory calculations of 1,2-dianilinosqairane.
J Phys Chem 2010;114:10097e109.
[21] Kamat PV, Das S, Thomas KG, George MV. Photochemistry of squaraine dyes.
1. Excited singlet, triplet, and redox states of bis[4-(dimethylamino)phenyl]
squaraine and bis[4-(dimethylamino)-2-hydroxyphenyl]squaraine.
J Phys
Chem 1992;96:195e9.
dyes [1,2,21]. Such
a
quenching process is less significant
(k ꢂ 108
M
ꢁ1 sꢁ1) in the case of the aminosquaraine dyes.
[22] Law K-Y. Squaraine chemistry: effects of structural changes on the absorption
and multiple fluorescence emission of bis[4-(dimethylamino)phenyl]squar-
aine and its derivatives. J Phys Chem 1987;91:5184.
4. Conclusions
[23] Park S-Y, Jun K, Oh S-W. The novel functional chromophores based on
squarylium dyes. Bull Korean Chem Soc 2005;26:428e32.
[24] Izutsu K. Electrochemistry in non-aqueous solutions. Viley-VCH Verlag
GmbH&Co; 2002.
The introduction of strong electron-withdrawing groups in the
p-position of the phenyl ring (for example: nitro) of 1,3-
bis(phenylamino)squaraines results in a colour change from yel-
low to red. The excited singlet state of aminosquaraines undergoes
radiative and non-radiative deactivation processes. The rate con-
stant for the radiative process depends on the viscosity of the sol-
vent. However, viscosity of the solvent does not affect the rate of
non-radiative deactivation. Fluorescence lifetime is short and
equals about 3 ns. The introduction of a nitro group to the p-posi-
tion of the phenyl ring results in a 5 times decrease of fluorescence
lifetime, in comparison to other dyes of the same aminosquaraine
series. It was also found that the fluorescence lifetime can be
correlated to the fluorescence quantum yields for these dyes in 1-
methyl-2-pyrrolidinone.
[25] Xiao P, Dumur F, Bui T-T, Goubard F, Graff B, Morlet-Savary F, et al. Pan-
hromatic photopolymerizable cationic films using indoline and squaraine dye
based photoinitiating systems. ACS Macro Lett 2013;2:736e40.
[26] Mori T, Miyachi K, Kichimi T. Electrical and luminescent properties of color-
changeable organic electroluminescent diode using squarylium dyes. Jpn J
Appl Phys 1994;33:6594.
[27] Thomas KG, Thomas KJ, Das S, George MV, Liu D, Kamatc PV. Photochemistry
of squaraine dyes. Part 10. Excited-state properties and photosensitization
behaviour of an IR sensitive cationic squaraine dye. J Chem Soc Faraday Trans
1996;92:4913e6.
[28] Olmsted J. Calorimetric determinations of absolute fluorescence quantum
yields. J Phys Chem 1979;20:2557e82.
[29] Vieira Ferreira LF, Costa SMB, Pereira EJ. Fluorescence quantum yield evalu-
ation of strongly absorbing dye solutions as a function of the excitation
wavelength. J Photochem Photobiol A Chem 1991;55:361e6.
ꢀ
[30] Je˛drzejewska B, Krawczyk P, Pietrzak M, Gordel M, Matczyszyn K, Samoc M,
et al. Styryl dye possessing donor-p-acceptor structure e synthesis, spectro-
scopic and computational studies. Dyes Pigments 2013;99:673e85.
[31] Xiao H, Mei C, Wang Y, Li H, Qian S, Yin H, et al. Novel triphenylamine-corred
two-photon absorbing dyes for labeling of biomolecules. Mater Chem Phys
2011;130:897e902.
Acknowledgements
This work was supported by The National Science Centre (NCN)
(Cracow, Poland), Grant No. 2013/11/B/ST5/01281.